The present invention generally relates to an ion source and more particularly to an ion source for producing an ion beam to be utilized in the processing as well as fabrication of semiconductors, thin films and the like.
In recent years, tendency of an ion beam being utilized in etching apparatus and sputtering apparatus for processing and manufacturing functional thin films, semiconductors and the like becomes more and more flourish in accompaniment to an increasing demand for the fineness in the implementation of these devices.
Many of the ion sources for producing the ion beam destined for the applications mentioned above are so designed that an arc discharge is sustained by emission of thermal electrons from a filament to thereby produce plasma of a desired gas. However, the conventional ion source of this type suffers a serious disadvantage that the use life of the filament is so short that frequent replacement thereof is required. This is because the plasma has to be produced from a gas which exhibits a high reactivity in such applications as etching or the like.
The problem mentioned above may be solved by resorting to adoption of a high-frequency (radio frequency or rf) discharge and a microwave discharge for producing the plasma. The ion source of this type can be operated over an extended use life because the filament is no more employed. The ion source operating based on the high-frequency discharge has long been studied and is reported, for example, in C.J. Cook et al's article "Energy Anomalies Observed in Ion Beams Produced by RF Sources" of "Rev. Sci. Instrum. 25", (1962). As discussed also in this literature, it is known that the ion beam extracted from a plasma produced through the high-frequency discharge has an energy level which is higher than the extracting potential by several hundred eV on an average (referred to as excess energy) and exhibits a considerably broad energy spread.
On the other hand, the inventors of the present application have experimentally discovered that the number of ions contained in the ion beam of an energy level not higher than 1 KeV extracted from the plasma produced through the high-frequency (rf) discharge is small when compared with that of the ion beam extracted from the plasma produced through the arc discharge or microwave discharge and having a density equivalent to that of the plasma produced by the high-frequency discharge mentioned above.
The ion sources in which the microwave is utilized are disclosed in detail, for example, in N. Sakudo et al's article "Microwave Ion Source" of "Rev. Sci. Instrum.", Vol. 48, No. 7, (July, 1977). The ion source of this type has a narrow energy spread and produces no excess energy, differing from the case of the high-frequency discharge type ion source. By virtue of these features, the microwave ion source can be advantageously used as the ion source for the ion implantation process involving mass separation. However, density distribution of the plasma produced by the microwave ion source does not always have a desired uniformity. When the ion beam of an energy level on the order of several hundred eV is extracted from the plasma produced by the microwave ion source for an etching process, the ion beam has an intensity distribution substantially proportional to the distribution of the plasma density. Accordingly, with the etching apparatus employing the ion source of the microwave discharge type, difficulty is encountered in realizing a uniform etching rate over a large area. For realizing a large-diameter beam (i.e. an ion beam having a large area), the plasma chamber may be constructed in a large size. In that case, however, it is necessary to establish the impedance matching for injection of microwave power into the plasma chamber, imposing thus restriction on the selection of dimensions of a waveguide as well as the plasma chamber. Further, in practical applications, restriction is also imposed on the size of a cylindrical coil for generating a magnetic field.